You can use the variable $vbulletin->userinfo["lastvisit"] which returns a timestamp from the user's last forum visit. From there, select posts with a timestamp GREATER THAN that. An example query (very, VERY basic) would be...
Code:
SELECT * FROM post WHERE dateline > ". $vbulletin->userinfo["lastvisit"] ." ORDER BY dateline DESC LIMIT 0, 10
That would display the 10 newest posts since the user's last visit. If you have a prefix set you would have to use that in front of the table names as well.